Electrochemical cell

ABSTRACT

An electrochemical cell having a bimodal positive electrode, a negative electrode of an alkali metal, and a compatible electrolyte including an alkali metal salt molten at the cell operating temperature. The positive electrode has an electrochemically active layer of at least one transition metal chloride at least partially present as a charging product, and additives of bromide and/or iodide and sulfur in the positive electrode or the electrolyte. Electrode volumetric capacity is in excess of 400 Ah/cm 3  ; the cell can be 90% recharged in three hours and can operate at temperatures below 160° C. There is also disclosed a method of reducing the operating temperature and improving the overall volumetric capacity of an electrochemical cell and for producing a positive electrode having a BET area greater than 6×10 4  cm 2  /g of Ni.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. W-31-109-ENG-38 between the U.S. Department of Energy andThe University of Chicago representing Argonne National Laboratory.

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 830,719, filed Feb. 4, 1992, which was acontinuation-in-part application of application Ser. No. 774,204, filedOct. 10, 1991.

BACKGROUND OF THE INVENTION

This invention relates to an electrochemical cell and to methods offabricating the cell and improving its capacity and/or power andcapability of operating at low temperatures. The invention also relatesto a new positive electrode or cathode during discharge forelectrochemical cells and method of fabricating same, and moreparticularly, relates to electrochemical cells and positive electrodesfor metal chloride batteries having lower internal impedance and greaterdischarge capacity with a higher specific energy and power.

According to the invention, an electrochemical cell comprises an alkalimetal, and preferably, a sodium negative electrode or anode duringdischarge which is molten at operating temperatures of the cell, analkali and preferably, a Na⁺ ion conducting solid electrolyte/separator,a molten salt liquid electrolyte in the positive electrode compartmentwhich is compatible with the positive electrode, and which is also atleast partially molten at the operating temperature of the cell, and apositive electrode which is impregnated by the liquid electrolyte andwhich comprises, as the electrochemically active positive electrodesubstance of the cell, a transition metal chloride which preferably isselected from the group consisting or iron chloride, nickel chloride,chromium chloride, cobalt chloride and manganese chloride or mixturesthereof. Since the cell with a Na electrode has received the majordevelopment effort, a shorthand method of referring to these cells is(Na/MCl₂) battery or electrochemical cell, wherein M is one of thetransition metals identified above. Batteries of this type are disclosedin U.S. Pat. No. 4,288,506 issued Sep. 8, 1981, to Coetzer et al. andU.S. Pat. No. 4,546,055 issued Oct. 8, 1985 to Coetzer et al. and U.S.Pat. No. 4,592,969 issued Jun. 3, 1986 to Coetzer et al. The batteriesor electrochemical devices of the type herein discussed are useful as apower source alternative to petroleum engines and are being developedcommercially, not only for electrically powered vehicles, but also forload leveling in electrical utilities.

An ideal electrochemical cell or battery should exhibit a number ofcharacteristics, including low resistance and high discharge rates,operation over a wide temperature range, a capability to operate over alarge number of cycles, and high energy on a volume, weight and cellbasis. Generally, these types of electrochemical cells or batteriesconsist of two dissimilar metals in an ionically conductive medium, withthe ionization potential of one metal sufficiently higher than the othermetal to yield a voltage upon reduction/oxidation redox (coupling) overand above that needed to break down the electrolyte continuously at thepositive electrode.

Metal typically goes into solution at the negative electrode or anode,releasing electrons to travel in the external circuit to the positiveelectrode, or cathode, doing work in transit. Material which will gothrough a valency drop on electrochemical discharge is included in thepositive electrode. In essence, this material, the oxidizer, acceptselectrons coming from the negative electrode and serves as thedepolarizer. The depolarizer or cathode is positioned, in oneembodiment, in the positive electrode in combination with someelectrolyte-containing matrix, and should be porous to allow access ofthe electrolyte to the enlarged area of the depolarizer or cathode.Porosity of the cathode provides a surface at which the redox reactionmay take place.

The economic and social advantages of powering automobiles frombatteries are considerable as the vehicles could operate at relativelyhigh efficiencies, such as 30-40%, and be non-polluting. Two importantcharacteristics are considered in seeking an energy storage system for avehicle. One of the characteristics or variables, specific power,designated in watt per kilogram (W/kg), determines to a large extent,acceleration and speed capabilities. The other consideration or variableof specific energy is designated as watt hours per kilogram (Wh/kg),determines vehicle range. The capacity density of a cell, or how muchelectrochemical energy the electrode will contain per until volume isdesignated as ampere hours per cubic centimeter (Ah/cm³).

It is generally seen, therefore, that increasing the cell capacityavailable during discharge and the cell power by lowering the internalimpedance of the cell are both important attributes in the considerationof how and when and to what extent electrochemical cells will be placedin the vehicle as a significant portion of the vehicle propulsionsystems.

Sodium/metal chloride cells of the type disclosed in the patentshereinbefore identified use a sodium anode, a β" alumina solidelectrolyte and a cathode designated as MCl₂ with a molten electrolyteof sodium chloroaluminate, NaAlCl₄.

Metal halide batteries exploit the higher electrolysis threshold valuesof the electrolyte constituents. In charging, the positive electrodebecomes poor in sodium salt with sodium metal being deposited on thenegative electrode and the halogen electrochemically reacting with themetal to form a metal halide. Among halides, the fluorides and chloridesexhibit higher electrolysis thresholds than bromide and iodides, andtherefore are preferred and generally used. As such, metal chloride andmetal fluoride systems exhibit relatively higher energy densities andlighter mass than systems using bromides and iodides. Because of thebetter electrochemical properties and low price, the metal chloridesystems are preferred.

As with other electrochemical cells, metal halide batteries generateelectricity by transporting electrons from the fuel constituent to theoxidizer, with concomitant oxidation and reduction occurring at thenegative electrode or anode and the positive electrode or cathode,respectively. The following reaction occurs:

    MX.sub.2 +2 Na←→2 NaX+M

where M is a transition metal and preferably is one or more of nickel,iron, cobalt, chromium and manganese and X is a halogen, preferablychlorine. The left hand side of the above equation depicts a chargedstate, before reduction of the metal halide, with the right hand side ofthe equation depicting a discharged state with reduced transition metal.

Utilization of the metal/chloride system is usually expressed on thebasis of the ratio of the reacted NaCl to the total quantity of NaClused to fabricate the positive electrode. This practice is convenientfor the Na/MCl₂ cell because they are fabricated in the discharge stateand the MCl₂ active material is formed electrochemically, as noted inthe above cell reaction. As used hereinafter, weight percent of aconstituent in the positive electrode refers to the positive electrodein the dry state, as the electrodes exist prior to being placed in theelectrochemical cell and cycled to charge the cell.

One of the significant problems in the sodium metal halide batteries isthe limited battery capacity, due to the chloride of the positiveelectrode metal which forms a layer of low conductivity on the positiveelectrode. Since this metal chloride has limited conductivity, after itreaches a certain thickness on the order of one micrometer, itpractically terminates further charge uptake of the cell. It has alsobeen noted that cell capacity may be lowered after repeated charge anddischarge cycles. Previous efforts to improve cell performance haveinvolved the addition of sulfur to the liquid electrolyte or theaddition of sulfide to the porous positive electrode. Neither of thesesolutions has been totally satisfactory.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a positive electrodeand electrochemical cell including same which overcomes the limitedcapacity of prior cells and permits enhanced charge uptake.

Another object of the invention is to provide an electrochemical cell ofthe alkali/metal transition metal halide type which has increasedspecific energy and power output due to lower internal impedance.

One feature of the present invention is the use of bromide and/or iodidecontaining additives in the positive electrode compartment to increasecell capacity and power.

Another object of the invention is to use certain pore formers in thecathode in combination with the bromide and/or iodide additions asdescribed herein providing improved electrode morphology and lowerimpedance resulting in lower cell operating temperatures.

Yet another object of the invention is to provide improved cell capacityand specific energy and power particularly with lower internal impedancedue to the use of a bromide and/or iodide additives, pore formers in thecathode and sulfur present either in the electrolyte or in the cathodeor both.

In brief, the objects and advantages of the present invention areachieved by providing electrochemical cells with various combinations ofadditives including bromide and/or iodide and sulfur containingmaterials and pore formers for electrode fabrication.

The invention consists of certain novel features and a combination ofparts hereinafter fully described, illustrated in the accompanyingdrawings, and particularly pointed out in the appended claims, it beingunderstood that various changes in the details may be made withoutdeparting from the spirit, or sacrificing any of the advantages of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of facilitating an understanding of the invention, thereis illustrated in the accompanying drawings a preferred embodimentthereof, from an inspection of which, when considered in connection withthe following description, the invention, its construction andoperation, and many of its advantages should be readily understood andappreciated.

FIG. 1 is a schematic view of one embodiment of the invention in theform of an electrochemical cell.

FIG. 2 is a graphical illustration of the relationship between thearea-specific impedance and the discharge capacity of the cell for cellswith no additives and cells with a 2 weight percent sulfur additive;

FIG. 3 is a graphical illustration of the relationship between thearea-specific impedance and discharge capacity like FIG. 2 for variouscombinations of additives to for the basis of the invention; and

FIG. 4 is a graphical illustration of the relationship between cellvoltage and discharge capacity for a cell without additives and a cellwith additives;

FIG. 5 is a graphical illustration of the relationship between the celldischarge energy and charged time;

FIG. 6 is a graphical illustration of the relationship between cellvolumetric capacity and cell operating temperature;

FIG. 7 are photomicrographs of the electromicrostructure comparing twoelectrodes; and

FIG. 8 is a table comparing four different nickel electrode formations.

DETAILED DESCRIPTION OF THE INVENTION

While the invention is primarily described with respect to asodium/transition metal chloride cell, it is to be understood that theinvention includes cells from other alkali metals, such as lithium andpotassium, with the electrolyte being changed to correspond to theparticular alkali metal.

Referring now to FIG. 1 of the drawings, there is disclosed asodium/metal chloride cell in schematic illustration. A single cell 10is illustrated, it being understood that a plurality of such cells maybe connected in series, as well as in parallel, to provide the requiredvoltage and battery capacity for any specific use such as powering anelectric car, or the like. The electrochemical cell 10 includes an outercasing 11 of any suitable container which can act as a negativeelectrode, the container may be steel or any other suitable electronconducting material. Alternate metals may be nickel or stainless steel,it being understood that any good electrical conductor which does notreact with the negative electrode material, which in this case issodium, may be used as an outer casing. The outer casing has a negativebuss or terminal 12 electrically connected to the casing positioned atthe top of the cell 10. A positive electrode or cathode 13 includes asolid rod of a transition metal and preferably nickel or iron orchromium or cobalt or manganese or any combination of alloys thereofwhich acts as a current collector and leads to a positive buss 14 at thetop for connection as desired. The solid rod 13 is surrounded bypositive electrode material 15 which is a combination of the chloride ofthe solid rod 13 and in the partially discharged state sodium chlorideand an electrolytic material, such as sodium chloroaluminate, NaAlCl₄.With lithium or potassium as the negative electrode, the electrolytewould be LiAlCl₄ or KAlCl₄, respectively.

For illustrative purposes, the positive electrode may contain nickel,nickel chloride, sodium chloride and sodium chloroaluminate which isalso liquid at cell operating temperatures which are generally in therange of from about 200° C. to about 400° C., but normally prior artcells operate in the range of from about 250° C. to about 335° C. A β"alumina electrolyte solid tube 16 is positioned to contain the rod 13and the positive electrode material 15 consisting of the chloride of therod 13 along with the sodium chloroaluminate. Outwardly of the β"alumina electrolyte 16 is the negative electrode 17 of sodium metal,which is liquid at cell operating temperatures. Finally, the cell 10 isclosed by an alumina header 18 in the form of a disc. The cell reactionsfor a positive electrode of NiCl₂ or FeCl₂ as hereinbefore stated are:

    NiCl.sub.2 +2 Na←→2 NaCl+Ni 2.59 V

    FeCl.sub.2 +2 Na←→2 NaCl+Fe 2.35 V

As hereinbefore stated, the positive electrode may be of a variety ofmaterials or transition metals, specifically the materials may includeiron, nickel, cobalt, chromium, manganese, or alloys thereof. While onetransition metal is normally used for the positive electrode,combinations may have some advantages. As an illustration, iron powdermay be used with a nickel rod as a current collector with FeCl₂ as themetal chloride in the molten electrolyte. For purposes of example only,and without limiting the scope of the invention, the nickel/nickelchloride positive electrode will be described. In all cases, thenegative electrode or anode was sodium metal, liquid at cell operatingtemperatures. In addition, sodium chloroaluminate was used with the β"alumina as the electrolyte material 16. To provide enough capacity andform sufficient quantity of nickel chloride, sodium chloride must beadded to the positive electrode during fabrication. Up to 0.66 gNaCl/gNiratio and high surface area of the nickel particles are required toachieve high capacity density (mAh/cm³). With the 0.66 gNaCl/gNi ratio,up to 33-50% electrochemical utilization of nickel is possible. Forcells with lithium or potassium electrodes, the salt would be LiCl orKCl.

Also, as hereinbefore stated, the nickel/nickel chloride positiveelectrode has a capacity and specific power which is limited by thenickel chloride layer formation on the surface of the nickel particles,which nickel chloride layer forms when the cell is charged. After thenickel chloride reaches a thickness on the order of about onemicrometer, further charge uptake is terminated. In order to remedy thisinherent limitation and improve the capacity and power characteristicsof the nickel/nickel chloride cell, the following additives andpreparation techniques were examined.

In general, the additives found beneficial contained bromide, iodide,sulfur, and various pore formers. More specifically, it was found thatbromide could be present in the range of 1 to 25 wt % expressed on thebasis of the weight of the positive electrode and as equivalent to NaBron the basis of halogenoid content, and/or iodide could be present inthe range of 0.05 to 25 wt % expressed on the basis of the weight of thepositive electrode and as equivalent to NaI on the basis of thehalogenoid content. The preferred amounts of NaBr and NaI or theirequivalents on halogenoid basis used were 5-10% and 5-12% by weight,respectively, but in any event, the total amount of halide should notexceed about 30% by weight. It is to be understood that other sources ofthe bromide and iodide may be used, such as AlBr₃, AlI₃, NiBr₂ and PbI₂,with the preferred amounts being in the above ranges based on the sodiumsalts.

Sulfur can be added to the positive electrode as elemental sulfur orsulfide, such as Na₂ S; the useful range for sulfur is 0.05 to 10 wt %with about 2% by weight being preferred. Where various combinations ofhalide and sulfur are used as additives, preferably the combinationshould not exceed about 30% by weight.

The pore former may be any material which decomposes to gases duringfabrication. The preferred pore formers are the ammonium salts ofcarbonic acid or other weak organic acids, such as formic, acetic oroxalic or these weak organic acids themselves since these do no causeundesirable reactions with the materials of the cell. Other materialssuch as oxamide or methylcellulose may also be used as pore formers, butthe preferred pore former is (NH₄)₂ CO₃. The pore former may be presentin the range of from about 5% to about 20% by weight, the preferredrange being about 5% to about 15% and the best results being about 10%by weight.

It is believed that the superior results reported herein are due, inpart, to the modification of the chloride coating and to the controlledpore distribution during positive electrode fabrication, therebyincreasing cell performance. This increased performance is primarilyevidence in capacity and/or power which may occur by a result ofdecreased impedance or by increasing the amount of active electrodematerial during the charge cycles. While the other alkali and transitionmetals may be used for cells of the invention, the results areparticularly favorable for the Na/NiCl₂ cell.

Where nickel felt or foam was used instead of nickel powder, the feltcould be used alone or additional nickel powder sintered to the feltcould be used. The pore former was always used with a sintered nickelelectrode, and weight percents for the pore former relate to the amountof the pore former before sintering.

Referring now to FIG. 2, the relationship between cell impedance anddischarge capacity is illustrated for a sodium/nickel chloride cellhaving no additives and no pore formers, it is seen that the cellimpedance of curve A in FIG. 2 sharply rises at relatively low dischargecapacities to provide a relatively unsatisfactory cell. Curve B of FIG.2 shows the slightly improved results when sulfur (2 wt %) is added tothe liquid electrolyte but the impedance is still very high at arelatively low discharge capacity.

In the examples as described below, positive electrodes were fabricatedas described for each example and installed in a cell as illustrated inFIG. 1 with a sodium negative electrode, solid electrolyte, and a NaCland NaAlCl₄ electrolyte which is molten at the operating temperature of300° C. A voltage of up to 3.1 V was applied to charge the cell with thecapacity, power and/or impedance measured as illustrated in FIG. 3-4 orfor the charging time illustrated in FIG. 5. Performance duringdischarge was also measured. Repeated cycles of charge, and dischargewere carried out.

EXAMPLE 1 7 wt % NaBr+10 wt % Pore Former

This examples illustrates the typical fabrication of Ni/NiCl₂ electrodewith the performance shown in FIG. 3, plot 1. The weight of thematerials are relative to the dry electrode weight 4.3 g nickel powder(0.68 m² /g BET area, 0.55 g/cm³ bulk density, 1.74 g sodium chloridepowder with mesh size -270+325, 0.605 g sodium chloride with mesh size-325+400, and 0.500 g sodium bromide (325+400 mesh size) were mixedtogether thoroughly. To this mixture of the salts a 0.7145 g of the poreformer, ammonium bicarbonate was added and thoroughly mixed. The mixturewas then placed in a stainless steel die and pressed to obtain anelectrode with 2.85 cm diameter and 0.5 cm thickness. This electrode asdescribed above was then placed in a tube furnace and heated first at250° C. for 30 minutes under a hydrogen containing atmosphere (5%hydrogen+95% helium) in order to remove pore former as ammonia, water,and carbon dioxide electrode was removed from the furnace and placed inthe cell in the positive electrode compartment. The cell was charged anddischarged with the discharge performance being measured and illustratedin FIG. 3, plot 1. A comparison of FIG. 3, plot 1 with FIG. 2, curve A,reveals the improvement in performance provided by the addition of thebromide additive and use of the pore former.

EXAMPLE 2 7 wt % NaBr+1 wt % Vapor-phase Sulfidation

The positive electrode was fabricated by the same procedure as describedin Example 1. The amounts of the chemical used was also exactly as inExample 1, except that no pore former was used for this electrode and,therefore, the electrode was not heated at 250° C. Rather, the electrodewas heated directly at 700° C. for one hour. After removing theelectrode from the furnace it was sulfidized to 1.0 weight percent bysulfur vapor. The performance of this cell system is shown in FIG. 3,plot 2. A comparison of FIG. 3, plot 2 with FIG. 2, curve A, reveals theimprovement in performance provided by the addition of the bromideaddition and the /wt % sulfur.

EXAMPLE 3 7 wt % NaBr+10 wt % Pore Former+0.5 wt % Vapor PhaseSulfidation

NaBr (0.5 g) and pore former (1.45 g) were introduced the electrode asdescribed above for Example 1 and the electrode was sintered and thensulfidized by 0.035 g of sulfur. Tests on the cell demonstrated thatthis combination produced better capacity, cycle life and lowerimpedance than the combination described in Example 2. Morespecifically, the curve for this Example would be between FIG. 3, plot 1and plot 2.

EXAMPLE 4 7 wt % NaBr+2 wt % S in Electrolyte

NaBr (0.5 g) was introduced in the electrode as described and 2 wt %sulfur by the electrode weight (7.145 g) was incorporated to the liquidNaAlCl₄ electrolyte. This combination produced lower cell impedance andhigher capacity than the cell in Example 3. These results demonstratedthat the addition of sulfur to the electrolyte also resulted in animprovement in cell performance.

EXAMPLE 5 7 wt % NaBr+10 wt % Pore Former+2 wt % S in the Electrolyte

The positive electrode was fabricated in accordance with the proceduredescribed in Example 1. A 2 wt % sulfur by the electrode weight (7.145g) was mixed very thoroughly to the liquid NaAlCl₄ electrolyte by slowlyand carefully increasing the temperature to 200° C. After mixing thesulfur with the NaAlCl₄ electrolyte, the positive electrode was placedin the positive electrode assembly of a Na/NiCl₂ cell. The performanceof this cell system is shown in FIG. 3, plot 5. A comparison of FIG. 3,plot 5 with plot 1, reveals the improvement in performance provided bythe combination of the bromide addition, the pore former and sulfur.

EXAMPLE 6 0.5 wt % NaI+10 wt % Pore Former+2 wt % Sulfur

Pore former was introduced in the electrode during electrodefabrication. NaI (0.035 g) and sulfur (0.1429 g) were added to theelectrode or electrolyte. The combination produced better cell capacityand impedance than the cell in Example 5. More specifically, the curvefor this example would be between the curves for FIG. 3, plot 5 and plot8, and would reveal that the small addition of the iodide was veryeffective compared to the bromide addition of Example 5.

EXAMPLE 7 7 wt % NaBr+2 wt % NaI+10 wt % Pore Former

This combination was incorporated in the nickel chloride electrodeduring fabrication. The incorporation was achieved with or without thepore former, but the inclusion of pore former produced better results.

EXAMPLE 8 7 wt % NaBr+10 wt % Pore Former+5 wt % NaI in the Electrolyte

The positive electrode was fabricated in accordance with the proceduredescribed in Example 1 except lower sintering temperature of 550°-650°C. was used. The amounts of the chemical used was exactly the same as inExample 1. Before placing the electrode in the positive electrodeassembly, sodium iodide (0.3573 g) was added to the electrolyte. Afterthis step, the positive electrode was placed in the cell assembly. Theperformance of this cell system is shown in FIG. 3, plot 8.

EXAMPLE 9 7 wt % NaBr+10 wt % NaI+10 wt % Pore Former+5 wt % sulfur

The positive electrode Ni/NiCl₂ was fabricated in accordance with theprocedure described in Example 8. Before placing the electrode in cellassembly, a 5 wt % sulfur (0.3572 g) and 10 wt % sodium iodide (0.7145g) by the electrode weight (7.145 g) were added to the NaAlCl₄electrolyte. The electrode was then placed in the positive electrodecompartment of Na/NiCl₂ cell. The performance of this cell system isshown in FIG. 3, plot 9. A comparison of the curves for FIG. 3, plot 9and plot 8, reveals the improvement provided by the combination ofadditive plus the pore former.

EXAMPLE 10 10 wt % NaI+20 wt % Pore Former

1.36 g Ni (15 vol %), 0.552 g NaCl (-270+325 mesh size), 0.259 g NaCl(-325 mesh size), and 0.231 g NaI (-325 mesh) were mixed togetherthoroughly. To this mixture of the salts a 0.4804 g of the pore formerammonium bicarbonate was added and thoroughly mixed. The mixture wasthen placed in a stainless steel die and pressed to obtain an electrodewith 1.15 cm diameter and 1.0 cm long. This electrode, as describedabove, was then placed in a tube furnace and heated first at 250° for 30minutes, under a hydrogen-containing atmosphere (5% hydrogen+95% helium)in order to remove pore former as ammonia, water, and carbon dioxidegases and, finally, to 600° C. for one hour for sintering. The electrodewas removed from the furnace and placed in a cell having the positiveelectrode within the β"-alumina tube and sodium negative electrodeoutside the tube. A 2 wt % NaI (0.048 g) relative to the dry electrodeweight was added to the liquid NaAlCl₄ electrolyte. The cell was chargedand discharged and the performance of the cell provided data for a curvebetween curve 1 and curve 5 in FIG. 3.

EXAMPLE 11 1 wt % NaI+20 wt % Pore Former

The positive electrode was fabricated by the same procedure as describedabove in Example 10. In this example, however, 1 wt % NaI was used. Theelectrode was sintered in the same way as described in the example. Theperformance of this electrode was lower than the electrode described inExample 10, but was improved over the performance by FIG. 3, curve 1.

FIG. 3 correlates to the various examples above reported and shows thecontinued improvement in lowering impedance and expanding the capacityof the cell for each addition of additives. For instance, curve 1relates to the addition of additives. For instance, curve 1 relates tothe cell made as reported in Example 1 and the other curves, 2, 5, 8,and 9 each corresponds to the same number Example. It is clear that theExample 9 which includes 10 wt % sodium bromide, 2 wt % sodium iodide, 3wt % sulfur with the use of the ammonium bicarbonate pore former in theamount of about 10% by weight of the dry positive electrode provided thebest results for the tests. In all cases, percentages of additivesexpressed as weight percentages of the positive electrode relates to theweight of the positive electrode in the dry state, that is before beingsoaked with the electrolyte and changed through cycling.

FIG. 4 shows the relationship between cell voltage and dischargecapacity for curve C representing a cell without any additives and curveD representing a cell made according to Example 9. As can be seen, thecapacity is much improved using the cell of the invention compared to anelectrochemical cell without additives whatsoever.

FIG. 5 shows the relationship between the charging time in hours and thedischarged energy in mWh/cm² (milliwatt hours per centimeter square),the curve representing an electrode made according to example 9. As canbe seen from FIG. 5, a cell made according to the present invention canbe charged up to 200 mWh/cm² in about one half hour and by about 3 hours600 mWh/cm² can be charged, representing almost 90% of the final chargeattainable even after 12 hours of charging time. This is a significantadvantage over the prior art presently known wherein charging times forthe prior art automobile batteries are in the neighborhood of 8-15hours. Recharging a battery for an electric car in half an hour asopposed to 8 hours is an extraordinary improvement.

FIG. 6 shows the relationship between the volumetric capacity inmilliamp hours per centimeter cubed (mAh/cm³) and the operatingtemperature of the battery. It can be seen from FIG. 6, which representsthe battery with a positive electrode made according to example 9, thatsuch a battery can operate at 150° C. compared to the usual 250° C.-335°C. operating temperatures for batteries presently being used. Theadvantage of low operating temperatures and the batteries in theenvironment are significant. By operating the battery at lowertemperatures reduces the heat management problems inherent with anybattery of this type. Operating at temperatures of 335° C. increases thesolubility of the nickel chloride present in the battery and when nickelions exchange for sodium ions in the electrolyte, the internal impedanceof the battery rises and hence, the heat given off during dischargerises. Moreover, lowering the operating temperature of the batteryincreases the battery life by reducing the glass seal corrosion. Theglass seals usually used in these batteries between the metal andceramics of the cell tend to corrode and the lower the battery operatingtemperature, the slower the corrosion reaction, thereby extending thelife of the battery.

FIG. 7 shows the difference in morphology for electrodes made inaccordance with example 9 characterized as ANL 92 and an electrodewithout the pore formers or halide additives which is designated ANL 90.The high surface area of about 10.3 m² /cm³ of the example 9 electrode(ANL 92) includes the existence of micro pores in the range of betweenabout 0.005 and 0.5 micrometers as well as macro pores in the range offrom about 1 to about 80 micrometers. The simultaneous presence of bothmicro pores and macro pores, referred to as bimodal pore distribution,results in an improved morphology of the nickel chloride electroderesulting in a high BET area. The macro pores in the nickel matrix donot get blocked by the formation of sodium chloride crystals during thedischarge reaction which gives easy access of the active material to theelectrode which would have been blocked if the macro pores were notthere. The existence of the micro pores increases the high surface areaof the electrode which apparently results when combined with the macropores in increased specific capacity and volumetric capacity asillustrated in the table of FIG. 8.

FIG. 8 shows that the ANL 92 (example 9) electrode is approximately 250%better in both specific capacity and volumetric capacity than is theprior ANL 90 electrode without the pore former and halide additives.Because the volumetric capacity determines the available miles a car canoperate before charging and is related to the power output of thebattery, it is volumetric capacity which is the most telling statisticwhen judging performance of electric automobile batteries.

Another feature of the invention is that batteries presently availablefor electric car use have an initial power in the neighborhood of 100watts per kilogram (W/kg) but by the end of the discharge cycle, thepresently available batteries are usually operating at about 60 W/kg.The battery of example 9 has an initial power of about 200 W/kg and afinal power, that is at the end of discharge, of about 170 W/kgdemonstrating not only the 100% increase in battery initial power butjust as important almost a 300% increase in power at the end of thedischarge cycle. This feature provides a significant advantage forelectric car operation because the battery power at the end of thedischarge cycle is within about 15% of initial power, a substantialimprovement over batteries which are presently available.

Accordingly, it is seen with the battery of the present invention,specific capacity, volumetric capacity and specific power are allgreatly increased with respect to the best known prior art batteries ofthis type.

By reversing the physical position of the positive electrode or cathodeand the negative electrode or anode illustrated in FIG. 1, more powercan be generated. In such an example, the nickel chloride would beoutside the β" alumina tube and the outer container would be preferablynickel, whereas the inner rod would be any good electrical conductorsuch as iron or steel or any other metal which would not chemicallyreact with the liquid sodium positioned inside the tube.

It is also known that the thickness of the electrode has an effect oncell operation and varying the thickness of the electrode, will vary theimpedance within the cell; however, it is believed that the addition ofthe additives described herein, these being bromide, iodide and sulfurcontaining materials and use of a suitable pore former enhances thedischarge capacity of the cell and lowers the impedance.

While there has been disclosed what is considered to be the preferredembodiment of the present invention, it is understood that variouschanges in the details may be made without departing from the spirit, orsacrificing any of the advantages of the present invention.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An electrochemical cellcomprising a bimodal positive electrode, a negative electrode of analkali metal, and a compatible electrolyte including an alkali metalsalt molten at the cell operating temperature, said positive electrodecomprising an electrochemically active layer of at least one transitionmetal chloride at least partially present as a charging product, andbromide and/or iodide and sulfur containing additives in said positiveelectrode or electrolyte, said bromide and/or iodide additive beingpresent in an amount not greater than about 30% by weight based on theweight of said positive electrode and the sulfur additive being presentin an amount not greater than about 10% by weight of said positiveelectrode, the amount of additives being sufficient to provide electrodecapacity in excess of 400 Ah/cm³ by being incorporated into thetransition metal chloride layer formed during charging of the cell. 2.The electrochemical cell of claim 1, wherein a pore former is present inthe positive electrode prior to formation thereof in the range of about5-30 wt %.
 3. The electrochemical cell of claim 2, wherein the poreformer is present in an amount not to exceed about 20 wt % and is anammonium salt.
 4. The electrochemical cell of claim 3, wherein thetransition metal chloride layer contains said additives.
 5. Theelectrochemical cell of claim 3, wherein the electrolyte contains saidadditives for incorporation into said chloride layer during charging ofthe cell.
 6. The electrochemical cell of claim 1, wherein the amount ofthe bromide and/or iodide containing additive is in the range of about1-25 wt % and about 0.05-25 wt %, respectively, based on the weight ofthe positive electrode and equated to NaBr and NaI, the total of theweights being in the range of about 1-30 wt %.
 7. The electrochemicalcell of claim 6, wherein the sulfur containing additive is present inthe range of about 0.05-10 wt % based on the weight of the positiveelectrode with the total of the bromide and sulfur additives being inthe range of about 1-25 wt %, the total of the iodide and sulfuradditives being in the range of about 0.10-25 wt % and the total of thebromide, iodide and sulfur additives being in the range of about 1-30 wt%.
 8. The electrochemical cell of claim 6, wherein the alkali metal issodium, the alkali metal salt is sodium chloride, and the bromide andiodide containing additives are bromide and iodide additives.
 9. Theelectrochemical cell of claim 8, wherein the transition metal is nickel.10. The electrochemical cell of claim 9, wherein the additives aresulfur and bromide.
 11. The electrochemical cell of claim 9, wherein theadditive is iodide.
 12. The electrochemical cell of claim 9, wherein theadditives are a combination of an iodide and bromide and sulfur.
 13. Theelectrochemical cell of claim 1, wherein the MCl₂ electrode capacity isin excess of 500 mAh/cm³.
 14. The electrochemical cell of claim 1,wherein the cell is operable at about 150° C.
 15. The electrochemicalcell of claim 1, wherein the alkali metal is sodium, the alkali metalsalt is sodium chloride, the transition metal is nickel, and the bromideand iodide containing additives are sodium bromide and sodium iodideadditives.
 16. The electrochemical cell of claim 1, wherein the sulfurcontaining additive is present in an amount of about 0.05-10 wt % and apore former is present in the positive electrode prior to formationthereof in the range of about 5-20 wt %.
 17. The electrochemical cell ofclaim 1, wherein about 30% of the discharged energy can be recharged inabout 1/2 hour.
 18. The electrochemical cell of claim 1, wherein about90% of the discharged energy can be recharged in about 3 hours.
 19. Theelectrochemical cell of claim 1, wherein the positive electrode afterformation thereof has micro pores in the range of from about 0.05 toabout 0.5 micrometers and macro pores in the range of from about 1micrometer to about 80 micrometers.
 20. The cell of claim 19, whereinthe BET area of the positive electrode is greater than 6.0×10⁴ cm² /g ofNi and a volumetric capacity in excess of 400 mAh/cm³.
 21. A bimodalpositive electrode for an electrochemical cell, comprising a sinteredelectrochemically active layer of at least one transition metal chlorideat least partially present as a charging product and additives ofbromide and/or iodide and sulfur, wherein said additives are present inan amount up to about 30% by weight of said positive electrode andwherein said sintered electrode BET area is greater than about 6×10⁴ cm²/g of Ni.
 22. The positive electrode of claim 21, wherein both micropores in the range of from about 0.005 to about 0.5 micro meters andmacro pores in the range of from about 1 to about 70 micrometers arepresent.
 23. The positive electrode of claim 22, wherein the transitionmetal is Ni and both bromide and iodide additives are present with thesulfur additive.
 24. A method of reducing the operating temperature andimproving the capacity of an electrochemical cell over repeated chargeand discharge cycles, the cell comprising a bimodal positive electrodecomprising an electrochemically active layer of one or more transitionmetal chlorides reducible during discharge to the transition metal, anegative electrode of an alkali metal, and a compatible electrolytecontaining an alkali metal chloride at least partially as a product ofthe cell discharge, the method comprising the steps of fabricating thecell in a discharge state of the transition metal positive electrode,the alkali metal negative electrode and the electrolyte, and adding abromide and/or iodide containing additive along with a sulfur containingadditive to the electrolyte sufficient to improve the cell capacity whenincorporated into the transition metal chloride layer during charging ofthe cell, the electrochemical cell being capable of operation attemperatures below 180° C.
 25. The method of claim 24 including the stepof charging the cell to incorporate the additives into the chloridelayer.
 26. The method of claim 24, wherein the electrode has avolumetric capacity in excess of 500 mAh/cm³ and is operable attemperatures below 160° C.